1. Field of the Invention
The present invention generally relates to a complementary metal-oxide-semiconductor (CMOS) image sensor, and more particularly to a hole-based ultra-deep photodiode in a CMOS image sensor for automobile application.
2. Description of Related Art
CMOS image sensor (or CIS) has been widely used, for example, in a cell phone camera, a web camera, a surveillance camera, a toy or medical equipment. The CMOS image sensor can be further adopted in a severe environment such as an automobile application, in which the requirements to the image sensor are very demanding due to harsh operation conditions of the automobile. Some issues at least need be resolved to adapt the CMOS image sensor for use in the automobile application.
Firstly, a high sensitivity or signal-to-noise ratio (SNR) is crucial particularly for a night operation of the automobile, such that more detailed information can be used for decision making.
Secondly, a low dark current is essential to maintain a high dynamic range (HDR), reduce dark signal non-uniformity (DSNU) noise and reduce dark signal shot noise, due to a fact that the operation temperature of the automobile is commonly higher than other applications such as the cell phone camera.
Thirdly, low blooming is an indispensable requirement, especially at night, for a road scene that is usually high dynamic range type. The CMOS image sensor is required to have a good blooming control at ultra-bright region in order to ensure its neighboring dimly lit regions are not washed out by blooming charges from the ultra-bright region. With respect to certain high dynamic range scheme, in which photodiode (PD) integrations are different, the blooming of a longer integration PD could destroy the information in a shorter integration PD.
Fourthly, red information is particularly important in the automobile application as tail lights of the automobiles and traffic light have a strong red component. Moreover, the image sensor is even required to collect infrared (IR) and near-infrared (NIR) information outside the visible light spectrum for better decision making.
In the conventional CMOS image sensor, a signal is represented by electrons in each pixel, and all transistors used in the pixel are n-type metal-oxide-semiconductor (NMOS) transistors. The holes and electrons generated from absorbed photons are stored on p-type side and n-type side, respectively, of a photodiode in the pixel. After exposure, only the electrons are transferred via an NMOS transfer gate to an n-type floating diffusion (FD) node, where the electrons are converted to a voltage signal by an FD junction capacitor. The voltage signal is then relayed by subsequent circuit as an output of the pixel.
In order to improve the dark current and the blooming issues mentioned above, a hole-based photodiode, as shown in FIG. 1, is disclosed, for example, in a disclosure entitled “Low-Crosstalk and Low-Dark-Current CMOS Image-Sensor technology Using a Hole-Based Detector,” Digest of 2008 IEEE International Solid-State Circuits Conference, 60-61, by Eric Stevens, et al., the disclosure of which is hereby incorporated by reference.
The hole-based photodiode shown in FIG. 1 suppresses more dark current than the conventional electron-based CMOS image sensor. Specifically speaking, the bulk dark current may be branched out by a p-type substrate 10, which acts as blooming holes drain. Further, the dark current could be substantially reduced at the Si/SiO2 interface, for example, between a shallow trench isolation (STI) 12 and an N+ well 14, due to dopant aggregation, compared to dopant segregation at the same interface in the electron-based CMOS image sensor. Moreover, as the mobility of hole is much lower than that of electron, drift and diffusion current in the hole-based CMOS image sensor under a same electrical field and charge distribution are much smaller than the electron-based CMOS image sensor. Furthermore, the grounded p-type substrate 10 is a low potential drain to holes bloomed from the hole-based PD, so it provides a good blooming control, which is desirable for auto applications.
However, the depth of a p-type photodiode 16 in the hole-based CMOS image sensor is limited by the implanted depth of an N well 18 because of heavier n-type dopants compared with p-type dopants. For example, atomic mass of n-type Phosphorus is 30.97 or Arsenic is 74.92, while p-type Boron is 10.81. The shallow p-type photodiode 16, accordingly, cannot collect substantial electron-hole pairs to cover red/NIR absorption region. On the other hand, for the PD 16 with a given depth, the depth of the N well 18 is limited by crosstalk and blooming control. If it is too deep, diffusion charges will go to the neighboring pixels instead of being collected by the p-type PD.
For the foregoing reasons, a need has arisen to propose a novel CMOS image sensor that has improved red/NIR response and maintain the good blooming and crosstalk controls in FIG. 1.